The NAVASTAR Global Positioning System (GPS) is a United States Defense Department satellite-based radio-navigation system transmitting information from which extremely accurate navigational information can be computed including the time, the user's three-dimensional position anywhere on or near the Earth, and the user's three-dimensional velocity. When fully operational, the GPS is planned to employ 18 satellites evenly dispersed in three, inclined, 12-hour circular orbits chosen to insure continuous 24-hour coverage worldwide. Each satellite carries extremely accurate cesium and rubidium vapor atomic clocks providing timing information. Additionally, each satellite is provided clock correction and orbital information by Earth-based monitoring stations.
Each satellite transmits a pair of L-band carrier signals including an L1 signal having a frequency of 1575.42 MHz (also referred to as 1540 f0 where f0 is 1.023 MHz) and an L2 signal having a frequency of 1227.6 MHz (1200 f0). The L1 and L2 signals are biphase modulated by pseudo-random noise (PRN) codes. The PRN codes facilitate multiple access. Since each satellite uses different PRN codes, a signal transmitted by a particular satellite can be selected by generating and matching (correlating) the corresponding PRN code pattern. Additionally, the PRN codes facilitate signal transmit time measurements which can be made by measuring the phase shift required to match the code. Both of the carrier signals (L1 and L2) are modulated by a PRN code which is referred to as a precision (p) code. The p PRN code, which is intended for military purposes, is a relatively long, fine-grained, precision code having a clock rate of 10.23 MHz (10 f0). The L1 carrier signal is additionally modulated by a PRN code which is referred to as a clear/acquisition (C/A) code. The C/A PRN code, which is intended for rapid signal acquisition and for commercial purposes, is a relatively short, coarsegrained code having a clock rate of 1.023 MHz (f0) and a code length of 1023 bits (one millisecond). A full bit (chip) of C/A PRN code, phase delay corresponds to a distance of 293 meters. In addition to the PRN codes, both of the signals (L1 and L2) are, continuously, biphase modulated by a 50 bit per second, 1500 bit long, navigation data bit stream. The navigation data bit stream includes information as to the status and emphemeris of all satellites, parameters for computing the particular satellite clock, and corrections for atmospheric propagation delays.
Due to limitations of conventional mixers and filters, commonly, prior-art-type, GPS-satellite signal receivers receive and down convert (to base band frequency), in multiple steps, the frequency of an (L1 or L2) GPS-satellite signal. More specifically, absent suitable filtering, conventional mixers, fold over a GPS-satellite signal, image frequency noise, which, if not removed, degridates the receiver performance. (Conventional mixers, when driven by a local oscillator (L0) signal, down-convert to an intermediate frequency (IF) both the signal (and/or noise) at the L0 frequency plus the IF frequency and the signal (and/or noise) at the L0 frequency minus the IF frequency. The L0 frequency plus the IF frequency is commonly referred to as the sum frequency; and, the L0 frequency minus the IF frequency is commonly referred to as the difference frequency. The undesired one of the sum and difference frequencies is commonly referred to as the image frequency. It should be noted that the image frequency differs from the desired, satellite-signal frequency, by twice the IF frequency.)
To remove the image frequency noise, commonly, prior-art-type, GPS-satellite signal receivers use a filter connected (in the satellite signal path) ahead of the mixer. The filter is designed (with a stop band) to attenuate the level of the image frequency noise (of the associated mixer) and (a pass band) to pass, relatively unattenuated, the level of the satellite signal. Unfortunately, the state of the filter art is such that it is relatively difficult to build a filter having a stop band relatively close to a pass band, such as is required if a conventional mixer is configured to down-convert directly to base band (or near base band) frequency the frequency of a GPS-satellite signal. As a compromise, common practice is to down convert the frequency of GPS-satellite signal in multiple steps in which the frequency of the GPS-satellite signal is reduced in each step by a factor of from five to ten.
Illustrated in FIG. 1 of the drawing generally designated by the number 100 is the radio frequency (RF) portion of a typical, "three-stage," prior-art-type, GPS-satellite signal receiver. Receiver 100 is shown to employ an antenna, designated 110, for receiving a GPS-satellite signal. In addition, receiver 100 is shown to employ a filter 120, an (RF) amplifier 122, a mixer 124, and a local oscillator 126, the combination configured to down-convert to a first IF frequency, the frequency of the received GPS-satellite signal. Also, receiver 100 is shown to employ another filter 130, an (IF) amplifier 132, another mixer 134, and another local oscillator 136, the combination configured to down-convert from the first IF frequency to a second IF frequency, the frequency of the received GPS-satellite signal. Further, receiver 100 is shown to employ still another filter 140, another (IF) amplifier 142, still another mixer 144, and still another local oscillator 146, the combination configured to down-convert from the second IF frequency to base band (or near base band) frequency, the frequency of the received GPS-satellite signal. Finally, receiver 100 is shown to employ yet another filter 180 and an amplifier 182, the combination configured to filter and amplify the down-converted, GPS-satellite signal.
Typically, the first IF frequency is from one tenth to one fifth the frequency of the received GPS-satellite signal frequency; and, the second IF frequency is from one tenth to one fifth the first IF frequency. Filter 120 is designed (with a stop band) to attenuate the level of the image frequency noise of mixer 124 and (a pass band) to pass, relatively unattenuated, the level of the received, GPS-satellite signal. Similarly, filter 130 is designed (with a stop band) to attenuate the level of the image frequency noise of mixer 134 and (a pass band) to pass, relatively unattenuated, the level of the first IF frequency, received, GPS-satellite signal. Finally, filter 140 is designed (with a stop band) to attenuate the level of the image frequency noise of mixer 144 and (a pass band) to pass, relatively unattenuated, the level of the second IF frequency, received, GPS-satellite signal.
It should be noted that not only is receiver 100 relatively complex, but, the receiver requires the use of three local oscillators (126, 136, and 146). Unfortunately, each of the three local oscillators (126, 136, and 146) is relatively expensive, particularly oscillator 126, due to its relatively high operating frequency. Further, each of the three local oscillators (126, 136, and 146) dissipates a relative large amount of power.
A much less complex, "single-stage," Global Positioning System Course Acquisition Code Receiver is disclosed in the U.S. Pat. No. 4,754,465 of Charles R. Trimble, the RF portion of which is illustrated in FIG. 2 of the drawing generally designated by the number 200. Receiver 200 is shown to employ an antenna, designated 210, for receiving the L1 (1540 f0) GPS-satellite signal. In addition, receiver 200 is shown to employ a filter 220, an (RF) amplifier 222, a mixer 224, and a local oscillator 226, the combination configured to down-convert, directly, to four f0, the frequency of the received GPS-satellite signal. Finally, receiver 200 is shown to employ another filter 280 and an amplifier 282, the combination configured to filter and amplify the down-converted (four f0), GPS-satellite signal. Although a separate RF filter (220) is shown, the filtering function is performed by amplifier 222.
Mixer 224 is of the starved-L0, balanced-type (image-reject harmonic mixer) to cancel (reject) signals and noise at the mixer image frequency (1532 f0). Local oscillator 226 generates a 768 f0 signal the level of which is sufficient to cause mixer 224 to double the frequency of the signal to 1536 f0 for mixing with an L1 (1540 f0) satellite signal to down convert the frequency of the satellite signal directly to four f0. It is important to note that, although an RF filter (220) is shown, the purpose of the filter is not to attenuate the level of the image frequency noise. The image frequency noise is canceled (rejected) by mixer 224. Further, it is important to note that receiver 200 employs only one local oscillator (for reduced power consumption), and that that oscillator is relatively inexpensive, as it operates at a relatively low frequency. Unfortunately, the mixer (224) is relatively expensive.